1. Field of the Invention
The invention relates to a semiconductor device and, in particular, to a symmetrical inductor for differential operation.
2. Description of the Related Art
Many digital and analog elements and circuits have been successfully applied to semiconductor integrated circuits. Such elements may include passive components, such as resistors, capacitors, or inductors. Typically, a semiconductor integrated circuit includes a silicon substrate. One or more dielectric layers are disposed on the substrate, with one or more metal layers disposed in the dielectric layers. The metal layers may be employed to form on-chip elements, such as on-chip inductors, by current semiconductor technologies.
Conventionally, the on-chip inductor is formed over a semiconductor substrate and employed in integrated circuits designed for radio frequency (RF) band. FIG. 1 is a plan view of a conventional two-turn symmetrical inductor device. The inductor is formed on an insulating layer 110 on a substrate 100, comprising a first winding portion and a second winding portion symmetrical with respect to the dashed line 2 on the insulating layer 100. The first winding portion comprises semi-circular conductive traces 101 and 103 and the second winding portion comprises semi-circular conductive traces 102 and 104. The semi-circular conductive trace 103 is parallel to and located outside the semi-circular conductive trace 101. The semi-circular conductive trace 104 is parallel to and located outside the semi-circular conductive trace 102. Each semi-circular conductive trace has a first end 10 and a second end 20, in which the first end 10 of the semi-circular conductive trace 101 extends to connect the first end 10 of the semi-circular conductive trace 102.
To maintain geometric symmetry, the second end 20 of the semi-circular conductive trace 103 is electrically connected to the second end 20 of the semi-circular conductive trace 102 through a lower cross-connection (underpass) 111. Moreover, the second end 20 of the semi-circular conductive trace 104 is electrically connected to the second end 20 of the semi-circular conductive trace 101 through an upper cross-connection 113. The first ends 10 of the semi-circular conductive traces 103 and 104 have lateral extending portions 30 and 40 for inputting/outputting signals.
Currently, wireless communication chip designs frequently use differential circuits to reduce common mode noise, with inductors applied therein symmetrically. In the inductor devices shown in FIG. 1, the lower cross-connection 111 is relatively closer to the substrate 100 with respect to the upper cross-connection 113. Thus, the capacitive coupling between the lower cross-connection 111 and the substrate 100 is greater than that between the upper cross-connection 113 and the substrate 100. Moreover, since the lower cross-connection 111 is thinner than the upper cross-connection 113, the conductor loss of the lower cross-connection 111 also exceeds that of the upper cross-connection 113. As a result, such a symmetrical inductor device cannot effectively reduce common mode noise and may reduce the quality factor (Q value) in differential operation.
Thus, there exists a need in the art for an improved symmetrical inductor device design to reduce common mode noise and increase Q value.